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[0015]The second technique for implementing the invention is to calculate sets of alternate trees, which can be used whenever a presently employed tree fails. Preferably, the alternate trees are maximally independent (i.e., have a minimum number of common nodes and links) of the original set of trees to minimize correlation between the original trees and the alternate trees. This helps insure that the nodes and links in the alternate trees that are employed to replace the original trees will not be likely to fail themselves soon after their implementation in the network. Various algorithms can be employed to calculate the backup trees in such a manner that the trees are maximally independent of one another without being so independent that the backup trees contain an excessive number of links, which themselves, would increase the likelihood of tree failure. The independence of the sets of trees also improves the mean time between route discovery cycles for a given interruption rate and hence reduces the control overhead and the rate of data loss.
[0016]Once a set of backup paths or trees is computed using the aforementioned or other techniques and the network starts multicasting, it is necessary to replenish the backup path or tree set in such a way as to maintain some quality of service, i.e., to maintain the probability of interruptions below some threshold. This means that new backup paths or trees need to be computed by the time the probability of failure of the current set of paths or trees rises above a given thre
Problems solved by technology
Multicasting in ad hoc networks is more challenging than in the Internet, because of the need to optimize the use of several resources simultaneously.
Firstly, nodes in ad hoc networks are battery-power limited.
Furthermore, data travels over the air and wireless resources are scarce.
Secondly, there is no centralized access point or existing infrastructure (like in the cellular network) to keep track of the node mobility.
The mobility of routers and randomness of other connectivity factors lead to a network with a potentially unpredicta
Method used
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example 1
[0049]E1=Set of all edges in a graph G and I is the family of subsets of E satisfying the condition that none of them contains a circuit of the graph. Hence independent subsets of this graph are all subsets of trees on this graph.
example 2
[0050]E=Set of all edges in graphs G and G′ (see FIG. 2) where G′ is a copy of G (edge e′1 is a copy of e1 and so on). Two matroids which can be defined on this set are:
[0051]M1=(E, I1) where an “independent” set is a union of subsets of trees of G and G′. For example, an independent set in the collection I1, could be {e1, e2, e3, e4, e′5, e′6, e′7, e′2} and a set which would not belong to I2 would be {e1, e2, e7, e′8} since it has a circuit in it.}
[0052]M2=(E, I2) where an “independent” set is one which does not have both copies of any of the edges. An example of an independent set in I2 would be {e1, e′3, e′7} and a set which would not belong to I2 would be {e1, e′3, e′1} since it contains both copies of e1.
[0053]Hence if a subset of E belongs to both Matroids defined above, it will have to be a union of 2 trees, one in G and the other in G′. Moreover, the copy of an edge that belongs to the tree in G should not belong to the tree in G′. This observation indicates that when 2 edge...
example
[0069]Consider the graph G in FIG. 2. The problem is to find two disjoint spanning trees. First, it needs to be verified that this is possible. This is easily done by trying different combinations of 4 edges each; four edges are needed to form a tree for a graph with 4 nodes. One example would be {e1, e3, e5, e7} and {e2, e4, e6, e8}.
[0070]The effectiveness of arriving at a pair of disjoint trees using the Matroid Intersection Algorithm can be seen by going through the process for this simple graph. Suppose one starts by building just a tree T1 first and then removing links of T1 from the set of edges and then trying to build another tree T2. The following two sets would be obtained (see FIG. 4):[0071]T1={e1, e2, e3, e4}[0072]T2={e5, e6, e7}
[0073]Adding e′8 to T2 will create a circuit and hence is not acceptable. Now one would like to move some edges from T1 to T2 and add some new ones to T1, so that it still remains a tree.
[0074]Constructing an auxiliary digraph G′ (see FIG. 3) hel...
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Abstract
A routing protocol for a multicasting network, such as an ad hoc network, employs alternate tree or path computation algorithms that continually compute backup trees or paths that can be employed to replace failed trees or paths. The sets of alternate multicast trees or paths are preferably pre-calculated before a first tree or path fails to minimize delay in replacing a failed tree or path. Preferably, the algorithms are designed to compute the alternate multicast trees or paths in such a manner that they are maximally independent of the original set of trees and paths to minimize correlation between the original trees or paths and the replacement trees or paths and to possibly increase the useful time of the calculated trees. This helps insure that the replacement trees or paths will not be likely themselves to fail soon after failure of the original trees or paths.
Description
PRIORITY CLAIM UNDER 35 U.S.C. § 119(e)[0001]This application claims priority, under 35 U.S.C. § 119(e), on U.S. Provisional Application No. 60 / 286,491, filed Apr. 25, 2001.GOVERNMENT RIGHTS STATEMENT[0002]This invention was made with Government support from the National Science Foundation (NSF) under Grant No. 9980521, the Office of Naval Research (ONR) under Grant No. N00014-00-1-0564 and the AFRL under contract No. F30602-97-C-0133. The Government has certain rights in the invention.BACKGROUND OF THE INVENTION[0003]1. Field of the Invention[0004]The present invention relates in general to a method and system for multicasting communications using routing schemes in which multiple independent trees or paths are employed to minimize communications interruptions due to tree or path failures. The invention is particularly suited for use in ad hoc networks.[0005]2. Description of the Background Art[0006]An ad hoc network consists of a collection of mobile routers that are interconnecte...
Claims
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